I've noticed a huge difference between highway and city mileage on my mopeds. Last year I went to visit my parents in a nearby city about 120km (75 miles) away. I used my 1982 Honda PA-50 for the trip. Normally its 3 litre (.75 gallon) tank is good for about 60km (38 miles). It almost made it the whole way on one tank: about 110km (70 miles)!
My mileage observations (city driving):
1982 Honda PA-50II: 5L/100km (47MPG)
1978 Peugeot 103: 5L/100km (47 MPG) (this used to be better before it was stolen and crashed a few times last year).
1981 Jawa Supreme III: 6L/100km (39 MPG)
1972 Vespa Ciao: 3L/100km (78 MPG)